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The adverse impacts of extreme heat on human health when a concurrent daytime and nighttime heatwave (CDNHW) occurs are greater than when daytime or nighttime heatwaves occur individually, because of the reduced recovery time from heat exposure. This study projects increases in CDNHW over the whole of East Asia under two Representative Concentration Pathway scenarios (RCP2.6 and RCP8.5) and two Shared Socioeconomic Pathway scenarios (SSP1‐2.6 and SSP5‐8.5). The daily maximum and minimum temperatures, which are used to define a CDNHW, are calculated of 3‐hourly temperatures of 25 km horizontal resolution produced by 12 general circulation model and regional climate model chains participating in the Coordinated Regional Climate Downscaling Experiment East Asia phase 2 project. In Historical simulation (1981–2005), occurrence period and occurrence rate of CDNHW from April to September area‐averaged in East Asia are 10.9 days and 0.9%, respectively. In projections for the future (2071–2100), occurrence period and occurrence rate of CDNHW will be 3 weeks and 3.7% (RCP2.6), 2 months and 20.5% (RCP8.5), 2 months and 15.6% (SSP1‐2.6), and 3 months and 45.7% (SSP5‐8.5). In addition, it is expected that the CDNHW intensity will increase, and the spatial extent of CDNHW will be extended. Although a CDNHW lasting less than 3 days is the most common, the proportion of CDNHWs lasting more than 10 days, compared to the total CDNHW frequency, will increase to 1.2% (RCP2.6), 7.2% (RCP8.5), 6.1% (SSP1‐2.6), and 17.3% (SSP5‐8.5) from 0.2% (Historical). Both occurrence rate and intensity of CDNHW will increase to a relatively large extent in Indochina, East and West China, and India. If the current greenhouse gas emissions continue, East Asia will experience unprecedented heat stress because the frequency and intensity of CDNHWs, which rarely occur during present‐day, will increase significantly over all regions by the end of the 21st century. [ABSTRACT FROM AUTHOR]